1. Field of the Invention
The invention relates to a method for monitoring the operational state of a surface inspection system for detecting defects on the surface of semiconductor wafers.
2. Description of the Related Art
In the course of the preparation of semiconductor wafers to form substrates for the production of electronic components, surfaces of semiconductor wafers are examined for the presence of defects. The surface to be examined is usually the upper side surface of the semiconductor wafer, on which the formation of structures of electronic components is intended. In order to carry out the examination, scanning surface inspection systems may be used. These gradually illuminate the surface of the semiconductor wafer with a light spot of laser light and detect scattered light as a function of one or different solid angles (channels). The scattered light data obtained in this way allow information to be deduced about the position and size of defects which are present on the surface examined.
So that the information about the size of defects coincides as accurately as possible with the actual size of the defects, the surface inspection system is calibrated with the aid of a reference semiconductor wafer. U.S. Pat. No. 7,027,146 B1 describes a way in which reference semiconductor wafers can be produced. Reference semiconductor wafers are also available for purchase. A reference semiconductor wafer as described in U.S. Pat. No. 7,027,146 B1 has reference defects of different size, the number and size distribution of which is known, deposited on its surface. Polystyrene latex spheres (PSL spheres) are often used as reference defects. In the case of PSL spheres, the real diameter of the sphere observed corresponds to the reported size of the defect. If the reference defect does not have a spherical configuration, the size of the defect usually means its largest spatial extent.
If the surface inspection system is properly calibrated, it indicates the number, position and the size of the defects on the reference semiconductor wafer with an accuracy that varies in terms of size within a specified tolerance limit (calibration tolerance). The measurement data obtained may, for example, be processed as a histogram which represents the frequency of the defects as a function of their size. The processing of measurement data may be restricted to size intervals, so that measurement data which relate to defects with a size lying outside a size interval are not taken into account.
It is important to monitor whether a surface inspection system is in a proper operational state, and if appropriate to warn if the monitoring reveals anomalies. If anomalies occur, their cause must be investigated, and if appropriate the proper state of the surface inspection system must be restored. U.S. 2007/0030478 A1 describes a monitoring method which provides repeated examination of a reference semiconductor wafer in the course of the use of the surface inspection system. If the measurement data of an examination of the reference semiconductor wafer do not differ substantially from those which the surface inspection system delivers in the freshly calibrated state, the state of the surface inspection system is regarded as in order. However, the scope and sensitivity of the examinations described leaves something to be desired. For instance, by measuring the number of defects no information is obtained about the stability of the measurement of defect sizes as a function of time. A possible drift of the defect size assigned to a defect by the surface inspection system is not noticed, or is only noticed late. In this regard, the information content remains insufficient, even if additional information is gathered about the position of the maximum of the size distribution and its variation as a function of time.